The wort is vigorously boiled for 60 to 70 min. The wort kettle must therefore be equipped with a powerful heating arrangement. With changes in the kettle heating systems, in the course of time, the shape of the kettle has also changed. The types of kettle heating can be divided into kettles with direct heating by coal, gas, or heating oil、kettles with steam heating、kettles with hot water heating.
Directly heated wort kettles
The oldest form of kettle heating is coal firing. This form of direct burning of fuel beneath the kettle bottom is now only encountered very rarely in old brewhouses. As a result of the curvature of the kettle bottom the boiling wort boils briskly from the middle to the outside. Later such kettles were often converted to direct heating with gas or heating oil.
Steam heated wort kettles
The most common form of kettle heating nowadays is steam heating. To understand the use of steam a little explanation is first necessary.
Steam temperature and pressure
Water boils at 100°C and the steam produced has the same temperature. Every child knows that but it only applies under standard atmospheric conditions. If the pressure in the vessel is increased-e.g. if the vessel is closed, like a pressure cooker – the water boils at a higher temperature. Under vapor-saturated conditions, every boiling temperature = steam temperature is associated with a defined pressure.
This means that the higher the boiling temperature is, the higher the pressure, much higher pressures and temperatures are employed in kettles. On the other hand, at very low pressures (under pressure, vacuum) water boils at even very low temperatures. Thus boiling occurs at a pressure of 0.06 bar at 36°C and a pressure of 0.02 bar at 17°C.
In the wort kettle boiling temperatures above 100°C occur because of the height of the wort. With a 2.5 m high wort level, the wort at the bottom of the kettle is at an overpressure of 0.25 bar, which is equivalent to a steam pressure of 1.25 bar, and the Table shows that this corresponds to a boiling temperature of 106°C.
If the wort kettle is heated from the bottom. this results in the formation of steam bubbles over all the bottom of the kettle which rise upwards and cause the movement of the wort and the driving out of undesirable components. Steam is fed at an overpressure of 2 to 3 bar (=133 to 143C) into the steam jacket around the bottom of the kettle. The steam transfers its evaporation heat to the wort and condenses whilst the wort is brought to boiling point.
A higher pressure, and with it a higher temperature, causes problems because of the higher interface temperature then produced at the kettle bottom. There is a risk of wort particles burning on the surface and affecting the taste of the beer.
Equipping a steam heated kettle with double bottom
For the heating to operate well a few basic considerations are necessary. Steam is supplied to the kettle in a well-insulated steam pipe (1) to avoid heat loss. Beyond the steam inlet valve (2) there is a pressure reduction valve (3) which reduces the steam pressure to the permitted pressure. In the case of kettles commonly used this is an overpressure of 2 to 3 bar. This pressure reduction is essential because the kettle bottom would rupture if live steam at about 15 bar was introduced. The steam is fed into an insulated circular channel (4) and distributed uniformly to the steam jacket through several supply pipes (5).
The steam jacket is insulated on the outside so that there is the least possible heat loss. As a pressure vessel, the steam jacket has to be provided with a safety valve (7) and a manometer(9). At the start of boiling the air in the jacket must first be driven out by steam. For this purpose 1 to 4 thin air-release pipes are welded onto the upper part of the steam jacket and each of these opens through its own valve into the upper part of the brewhouse. These air release valves (8) remain open at the start of boiling and also during boiling until steam flows freely from them. It is then certain that the air is completely displaced from the steam jacket. At the end of boiling the valves are opened to prevent a vacuum from being formed as a result of steam condensation and causing the bottom of the pan to collapse. Nowadays this operation is performed automatically. There is a temperature difference between the wort and the steam which is evened out by the heat-conducting bottom of the kettle. The wort becomes heated until it boils whilst the steam delivers its heat and in doing so condenses.
The resulting condensate water is heavier than the steam and collects in the lower part of the steam jacket. The condensate water must be drained off through the condensate drainpipe (10) into the condensate pot (11). The condensate pot is located beneath the steam jacket and allows only water, but no steam through. Most condensate pots operate on the float principle.
The condensate water is led off through the condensate water drain pipe (12). The condensate water is pure water and is returned to the kettle as the kettle supplies water via a collection vessel.
When discussing the mash conversion vessel it was pointed out that the previously customary double steam bottom has largely disappeared nowadays and heating occurs through semicircular pipes welded onto the vessel bottom or side. The same applies equally to wort kettles. Wort kettles which are equipped with an internal or external boiler operating at low pressure arc no longer provided with a kettle heating system since the low-pressure boiler by itself provides the energy needed.
Kettle shape and material
The shape of the wort kettle has undergone many changes in the course of time. The first steam heated kettles were built with a spherical shape in 1890. Later a change was made so that the middle of the kettle bottom was raised higher in order to achieve a better boiling of the wort from the middle to the periphery.
The same effect was obtained with in-built auxiliary heaters and with kettles having internal zone boiling. In these, the wort was additionally heated by steam at a higher pressure (up to 5 bar 158°C) and evaporation accelerated. The necessary heating surfaces could not be increased in proportion to the grist load.
Around 1950 a range of brewhouses were constructed in block form in which the brewing vessels were situated above one another. The wort kettle, located at the bottom, had a rectangular ground surface but the bottom half was made semicircular so that the steam heating of the side would provide good circulation of the wort.
The compact design kettles which appeared in the sixties were made by welding together parts cut from smooth metal sheets. This did away with the expensive beating out of the beautiful copper hoods which still adorn many brewhouses. As a result of the different slope angles of the walls and the heating pipes, here too good circulation was obtained but it did not reach the corners completely. It was thereby possible to increase the heating surfaces in proportion to the grist load.
Hot water heating (hydro boiling)
It is also possible to heat water under pressure to a high temperature below boiling point and to use this hot water at 160 to 170°C to heat the wort kettle. In this way, there are no losses connected with the condensation of the steam. On the other hand, a substantially larger pipe diameter is needed and also more energy than steam, since steam is much easier to move than liquid water. Consequently, hot water boiling is found less often today than the usual steam heating. Heat is transferred to the wort through welded-on semicircular pipes as already described for the mash kettle.
Wort kettle with low pressure boiling
The basic idea of low pressure boiling is that a series of conversion processes proceed more quickly if the pressure and hence the boiling temperature are higher than 100°C.
Wort kettles with low pressure boiling are designed as sealable pressure kettles for a maximal overpressure of 0.5 bar and are equipped with the necessary safety fittings for over- or underpressure. Boiling of the wort occurs by means of an internal or external boiler. The kettle steam condenser is located in the pressure area of the kettle so that the higher vapour temperature can be utilized.
In the case of low pressure boiling, the wort is boiled for 50 to 60 min at 102 to 104°C. Total evaporation using low pressure boiling is in the region of 5-6 %. Boiling occurs either in an external boiler situated outside the kettle through which the wort is pumped or in an internal boiler where the wort is heated in the wort kettle.
Low pressure boiling with an external boiler
In the case of kettles with external boilers, the wort is circulated 7 to 8 times an hour through an external boiler situated outside the kettle. For this, the wort is continuously withdrawn from the bottom part of the kettle and pumped through the external boiler by a pump.
Shell-and-tube heat exchangers (tube evaporators) are the most commonly used form of external boiler, with the plate heat exchanger being used for this purpose less frequently. The wort is fed through the tubes which in turn are surrounded by steam from outside. Whilst the wort is being heated, the steam cools down and condenses. External boilers are built either in a standing or lying position, in the case of the latter, slightly sloping for better drainage of the condensate. Both forms are common.
The size of the external boiler (heat exchanger) is determined by the necessary surface to be heated. The heating surface is determined by the number of heating pipes and their diameter and length.
If the wort is allowed to flow through the pipes at a low flowthrough velocity there is the risk of the wort burning or at least caramelizing and hence increasing in color. Furthermore, there is the danger that as a result of too high temperatures, coagulated protein collects in the pipes. To prevent this, a higher flowthrough velocity of 2.6 to 3.0 m/s is usually required today. To achieve the same exchange of heat, each wort particle has to travel a longer way. Since the length of the boiler is, however, limited for reasons of space, the ends of the heat exchanger are frequently med as baffles, so that each wort particle passes through the heat exchanger several times. The baffle areas, however, cause shear forces for the wort particles. So as to have few baffle areas, the external boiler may be of considerable length.
Low pressure boiling with internal boiling
Modern wort kettles are very frequently built with an internal boiler. The internal boiler is a tubular heat exchanger in the wort kettle through the vertically arranged tubes of which the wort ascends, whilst being heated from the outside by steam. The steam which is introduced from the top thereby cools, condenses, and is drained off.
In a stemming cone, the boiling wort is accumulated above the wort level of the kettle and is centrifuged against a distributing shield, which spreads the wort out widely and ensures evaporation, whilst returning the wort to the wort level.
Whilst the temperature of the wort during boiling increases to 102 to 104°C, the temperature (and therefore the pressure) of the hot steam of course has to be considerably higher. It is on heating at about 140 to 145℃ and on boiling at about 130℃ (= 2.8 bar).
The wort streams into the heating tubes of the boiler from the bottom at a temperature below 100 °C and is heated as it ascends. This very soon results in the formation on the inner walls of bubble fur, which as it rises higher turns into subcooled nucleate boiling and finally in the largest zone becomes complete ebullition, whilst outside the steam releases its evaporation heat (enthalpy) and condenses. The condensate flows in an increasingly thick layer to the bottom thereby increasingly inhibiting the heat exchange.
In the case of ebullition, the water to a large extent turns into steam which has a far greater volume than the water from which it has been formed. The greater volume hereby produced is raised in a stemming cone above the heating tubes higher than the level of the wort in the kettle, and then with the aid of a distributing shield returned to the wort surface. The distributing shield, which can have different forms, is so regulated that a complete circulation of the wort in the kettle occurs without the occurrence of dead zones.
Conclusion
In conclusion, the evolution of wort kettles from direct coal firing to modern steam heating and low-pressure boiling reflects ongoing advancements in brewing technology. These innovations not only improve brewing efficiency and product consistency but also underscore the industry’s commitment to sustainability and operational excellence.
